Process and apparatus for the degassing of viscose



Dec. 18, 1956 A. BUURMAN 2,774,441

PROCESS AND APPARATUS FOR THE DEGASSING OF VISCOSE Filed Nov. 50, 1955 ATTORNEY United States Patent PROCESS AND APPARATUS FOR THE DEGASSING OF VISCOSE' Aart Bnurman, Velp, Netherlands, assignor to American Enka Corporation, Erika, N. C., a corporation of Delaware Application November 30, 1953, Serial No. 395,161

Claims priority, application Netherlands February 9, 1953 5 Claims. (Cl. 183-25) This invention relates to a method and apparatus for the continuous deaeration of viscose spinning solutions such as are employed in the manufacture of rayon yarns, films, foils and the like.

In order to produce good quality rayon products, it is necessary that the spinning solution be deaer-ated before delivery to the spinning pumps. Heretofore this deaeration has been brought about by delivering the spinning solution to an evacuated tank where the gas in the solution is evolved as a result of the reduced pressure. While this method is efiective to eliminate gas, the amount of foam produced is so great that the capacity of the system is quite low in relation to its size so that to handle a large quantity of viscose a tank of tremendous capacity is necessary.

It is an object of the present invention to overcome. the foregoing difiiculties and to provide for the continuous deaeration of spinning solutions without foam formation 7 and under conditions which permit the use of physically small, convenient and easily maintained apparatus.

It is proposed according to the present invention that the spinning solution to be treated be passed from the bottom up through a pipe having a substantial, vertical component and that the atmosphere at the top of the pipe be held under strongly reduced pressure. The viscose is flowed upwardly through the pipe at a speed such that the temperature and pressure conditions permit boiling at the bottom of the pipe while, as a result, of cooling, the solution no longer is boiling at the top of the pipe. At the top of the pipe the solution and the gases are separately recovered.

Other objects and advantages of this invention will be apparent from adiscussion of the following detailed description of two embodiments thereof in conjunction with the annexed drawings wherein:

Figure l is a schematic view partially in elevation and partially in section of a viscose deaeration system according to the present invention; and

Figure 2 is a view similar to Figure 1 of a modified deaeration arrangement according to the present invention.

In Figure 1 the numeral 10 represents a viscose storage tank which is connected by a pipe 11 to a heat exchanger 12. After passage through the heat exchanger 12, the viscose is delivered from the heat exchanger through a valve 13 to a vertical pipe 14 which leads upwardly to a low pressure chamber 15 containing therein a weir 16, a deaerated viscose discharge line 17 and a gas withdrawal conduit 18. In the heat exchanger 12 the viscose is warmed to a temperature below its boiling point at the existing pressure, i. e., about C. The supply to the pipe 14 is so regulated that below the valve 13, for example at the point 19, the pressure of the viscose is atmospheric or higher. A substantial vacuum is maintained in the container 15. Thus, in the pipe 14, the pressure will gradually decrease owing to the vacuum above the liquid level. Incident to the reduction in pressure, any air bubbles in the viscose will tend to increase in size. Furice thermore, when the pressure on the liquid gets below the vapor pressure of the solution, boiling will occur, and vapor bubble formation takes place around the air bubble nucleus. The rate of How upwardly in the pipe 14 is so regulated that the temperature drop caused by boiling will have cooled the viscose enough so that when it flows to the pipe 17, boiling has stopped.

It is convenient to make the pipe 17 about 9 meters long which will cause it todraw a barometric vacuum above pipe 14, i. e., within the chamber 15. If it is desired to produce the vacuum by other means, it is possible to connect a pump at the conduit 13 which willnot only withdraw the gas as fast as it is evolved but will maintain the desired degree of vacuum within the cham ber 15.

By use of the arrangement just described, foam formation in the pipe 14 is avoided and the viscose is rendered completely free of air as it spills over the weir 16. If the arrangement of Figure 1 is set up in glass and observed, it is noted that while the spinning, solution is slowly moving through the pipe 14, very small bubbles become visible. As the liquid moves upwardly, the bubbles increase bothv in size and in number and leave the solution at the end. of the pipe 14 and enter the chamber 15 while theliquid. spills over the weir.

The time during which the spinning solution must be kept in the pipe 14, the. speed at which it fiows therethrough and the length of' the pipe 14 have been found to differ to some extent on the nature of the spinning solution, particularly its viscosity. Nevertheless, general rules have been discovered which, whenfollowed, give completely satisfactory results and which, may well be considered to be of general applicability.

It hasbeen found that good results are, achieved when spinning solution remains in the pipe 14for a minute or more, moving at 0.5 to 1 meter per minutethrough. a pipe from 40 to centimeters long. The, preferred arrangement is to move the. solution at 0.5 meter a minute through a pipe 14, which isv 100 centimeters long. It has been discovered that heating the spinning solutions before deaeration is advantageous particularly if the solutions are of high viscosity. Temperatures from 30 to 60, C. may be used but it is best to restrict the, range to 40 to 55 C. Experimentally it has been noted that if the viscose entering the bottom of the pipe 14 is at about 40 0, its temperature has been reduced to about 18 C. by the time it spills over the weir. Itcanbe-seen that by suitable control of the conditions such as the extent of preheating, the rate of flow in the deaeration pipe 14 and the magnitude of the vacuum, it is possible to. discharge the viscose at a temperature quite a bit lower thanthat at which it is delivered to the bottom of the pipe. 14. Thus, even if preheating is used, no cooler is necessary. Furthermore, in the event that the spinning solution is sensitive to heat, the temperature change may be restricted to a small value and the maturity of the viscose is not afiected to an undesirable extent.

It is apparent that the process of the present invention is not dependent upon preliminary heating to make it operative. Hence, it may be used with spinning solutions containing solvents having boiling points below that of water. The temperature drop prevents, in the presence of a low boiling solvent, the undesirable evaporation of that solvent as a result of the reduced pressure. In other words, the cooling which occurs in the pipe 14 is desirable in the retention of solvents. V

In achieving the advantages of this invention, it is most important that the pressure temperature values between the viscose supply tank 10 and the heat exchanger 12, and, indeed, in the whole system ahead of the valve 13, be such as to prevent boiling before the solution passes through 3 the valve 13. For if foam forms in the pipe 11, it is found to persist within the vertical pipe 14. The movement of the liquid upwardly in pipe 14 is at such a rate that the bubbles go up with, rather than with respect to, the solution. If the bubbles formedare too large and tend to rise faster than the liquid, the deaeration pipe 14 should be baffled, as fol-example by the use of sieves of 0.3 to 0.5 centimeter mesh. 7

Referring now to Figure 2, the numeral20 represents a heat exchanger which is fed with viscose from a source, not shown, by a pipe 21. The warmed viscose is led through a valve 22 in a supply conduit 23 which terminates in a restricted orifice 24. The pipe 23 is disposed coaxially in a vertical pipe 25 which functions as the deaeration pipe. The pipe 25.terminates in a weir at 26 and has above it a low pressure chamber 27 from which vapors may be withdrawn through a conduit 28. The liquid spilling over the weir 26 flows out through a conduit 29 to use. The orifice at 24 is of such size as to keep the pressure within the pipe 23 at a value which will not permit of boiling at the prevailing temperature. Sieves 30 f 0.3 to 0.5 centimeter mesh act to retard the rise of vapor bubblesand to keep those bubbles from moving upwardly at a rate much faster than the spinning liquid.

Operation of the system of Figure 2 may be understood by reference to an example'in which viscose containing 6.5% of cellulose and 8% of NaOH was deareated. The viscose had a ball viscosity of 200 seconds at room temperature. The sample had 0.85% air before treatment and a Xanthate ratio of 0.57 In heat exchanger 20 this viscose solution was heated to 42 C. and it entered pipe 25 at the rate of 2000 cubic centimeters per minute supply a viscose solution to thebottom of-said conduit through the orifice at 24. The free cross section of the pipe 25 was 140 square centimeters and it was 100 centimeters high. The meshes 30 were 0.5 centimeter and the pressure in the chamber 27 was adjusted at 20 millimeters of mercury absolute. At the lower end of the pipe 29, 1920 cubic centimeters of viscose per minute was discharged and upon testing was found to contain less than 0.02% of both dispersed and dissolved air while having a xanthate ratio of 0.56. It was found that'water was evaporated at the rate of about 80 centimeters a minute and the cellulose and NaOH content of the viscose rose to about 6.75 and 8.3%, respectively.

While this specification has been concerned with viscose spinning solutions, it is adaptable to cupr-ammonium solutions and spinning solutions with organic solvents such as polyamides in benzene, polyacrylonitrile in dimethylformamide, cellulose acetate in acetone, copolymers with acrylonitrile components in various organic solvents.

What is claimed is:

1. The method of deaeration of viscose that comprises heating succeeding volumes of a viscose solution while zone at the top of the column to cause the solution entering the column to boil, flowing succeeding volumes of" the solution upwardly in the column until it has cooled enough so that boiling stops and thereafter spilling the viscose at the top of the column. to said zone of reduced pressure and recoveringthe'viscose from the bottom of saidzone.v .f

2. Apparatus for the deaeration of viscose that comprises means defining a chamber, a substantially vertical conduit extending below saidch amber' and with .its top' in communication therewith, means subdividing theicross section of said conduit for a part of its length, means to for upward flow into said chamber, means to heatthe viscose supplied to said'conduit, means to maintain said chamber at a pressure below atmospheric and low enough to cause boiling at the existing temperature of the viscose solution before the viscose reaches the top of the conduit and means to recover liquid from said chamber at a level below the top of said conduit.

3. Apparatus for the deaeration of viscose 'that comprises means defining a chamber, a substantially vertical conduit extending below said chamber and at its top in communication therewith, means to evacuate the chamber, means to supply viscose solution to the bottom of said conduit for upward flow into said chamber, means to heat the viscose solution supplied to the bottom of said conduit to a temperature high enough to cause boiling in said conduit before. the solution issues from the top of the conduit of the said chamber and means to recover liquid from said chamber.

4. The method of degassing spinning solutions that comprises, passing succeeding volumes of a spinning solution upwardly through acolumn and spilling the same into a zone at the top of the column while adjusting the temperature of the viscose entering the column and the pressure in said zone tofcause boiling to occur in the column but to terminate before the solution spills into said zone and recovering the degassed solution from said zone.

5. Apparatus as claimed in claim 2 in which the means subdividing the cross-section of the conduit are sieves.

References Cited in the file of this patent UNITED STATES PATENTS 414,618' Weiss Nov. 5, 1889 847,085 Lehnert Mar. 12, 1907 863,031 Lehnert Aug. 13, 1907 r 1,529,884 Hall Mar. 17, 1925 1,581,371 Weisgerber Apr- 20, 1926 1,752,215 Waters Mar. 25, 1930 1,926,191 Boucherot et al Sept. 12, 1933 2,006,985 Claude et al. July 2, 1935 2,355,057

Copeland Aug. 8, 1944 

4. THE METHOD OF DEGASSING SPINNING SOLUTIONS THAT COMPRISES, PASSING SUCCEEDING VOLUMES OF A SPINNING SOLUTION UPWARDLY THROUGH A COLUMN AND SPILLING THE SAME INTO A ZONE AT THE TOP OF THE COLUMN WHILE ADJUSTING THE TEMPERTURE OF THE VISCOSE ENTERING THE COLUMN AND THE PRESSURE IN SAID ZONE TO CAUSE BOILING TO OCCUR IN THE COLUMN BUT TO TERMINATE BEFORE THE SOLUTION SPILLS INTO SAID ZONE AND RECOVERING THE DEGASSED SOLUTION FROM SAID ZONE. 